EXPLANATION OF SCIENTIFIC TERMS 



MADE USE OF IN THIS VOLUME. 



ABERRATION ( Latin alerrare, tostray) 

 is simply any wandering of a body from 

 the path in which it is expected to move. 

 In Natural Philosophy, the term is used 

 with respect to the rays of light. In 

 Astronomy, for example, the apparent 

 place of a star differs from the true; be- 

 cause, light not being instantaneous in 

 its progress, the earth will have moved so 

 far in her orbit while the particle of light 

 which renders the star visible is passing 

 to the eye ; and, hence, the tube through 

 which we view the luminary must be 

 directed forward, on a similar principle 

 as the fowler points his gun before the 

 bird which he would shoot in its flight. 



, SPHERICAL. The 



intention of spherical lenses, or of con- 

 cave mirrors, is that the rays of light 

 should, in the former case, be refracted, 

 and in the latter reflected, so as to con- 

 verge and meet in a single point or focus. 

 In practice, the rays are generally found 

 to deviate from that point, and this devi- 

 ation is termed the Spherical aberration 

 of the lens, or of the mirror. These 

 aberrations proceed from two causes : 

 from the form of curvature of the lens, 

 or of the reflector, and from the different 

 refrangibility of the rays of light. See 

 Refra ng i bility. 



ABSCISSA. See Conic Sections. 



ACCELERATION is an increase in the 

 rapidity of the motion of a moving body. 

 Thus it being found, by experiment, 

 that a stone, or other body, falling to the 

 earth, moves faster and faster as it de- 

 scends, the motion is said to be conti- 

 nually accelerated. 



ACCELERATED FORCE is the in- 

 creased force which a body exerts in con- 

 sequence of the acceleration of its motion. 



MOTION. See Acce- 

 leration. 



ACCIDENTAL COLOURS. If we look 

 intensely with one eye upon any coloured 

 spot, such as a wafer placed on a sheet of 

 white paper, and, immediately after- 

 Avards, turn the same eye to another part 

 of the paper, we shall see a similar spot, 

 but of a different colour. Thus, if the 

 wafer be red, the seeming spot will be 

 green ; if black, it will be changed into 

 white; and, in the same manner, every 

 colour has a corresponding one into which 

 it is transformed. These corresponding 

 spots are termed Accidental Colours, or 

 Ocular Spectra. 



ACHROMATIC (from the Greek a priva- 

 tive, and chroma, colour) signifies wit/i- 

 out colour. Objects, when viewed through 



an ordinary telescope, appear to be co 

 loured round their edges, on account of* 

 the different refrangibility of the rays of 

 light ; and telescopes which are construct- 

 ed so as to counteract, or prevent, this 

 aberration, are denominated Achromatic. 



ACTING POINT. See Machine. 



ACTION is that motion which one body 

 produces, or endeavours to produce, in 

 another. Mechanical Action is exerted 

 either by percussion or by pressure ; and, 

 in either case, the force exerted by the 

 acting body is repelled in an equal degree 

 by the body on which it acts. Thus, in 

 driving a nail with a hammer, the stroke 

 acts as powerfully against the face of the 

 hammer as against tbe head of the nail ; 

 and, in pressing the hand upon a stone, 

 the pressure upon tbe stone is equally 

 impressed upon the hand. In each of 

 these cases the impulse is counteracted 

 by Avhat is termed the Re- action ; and 

 that " Action and Re-action are always 

 equal" is not only laid down as an axiom 

 in mechanics, but is understood to be a 

 general law of nature. 



ACTIVE FORCE. See Force. 



ACUTE ANGLE. See Angle. 



ADAMANTINE SPAR. See Corundum 

 and Spar. 



ADULARIA. See Feldspar. 



AERIFORM BODIES, or AERIFORM 

 FLUIDS. See Gas. 



AGATES are not simple stones, but ag- 

 gregates of different species, such as 

 quartz, flint, amethyst, &c. ; all differing 

 in colour and transparency, but sliding 

 into one another by almost imperceptible 

 gradations. The Mocho-stones, which 

 appear as if they contain little stems of 

 moss, are Agates ; and so are the varie- 

 gated Scotch pebbles. 



AIR, IN A POPULAR SENSE, is that 

 transparent invisible fluid which sur- 

 rounds the earth, and in which we move 

 and breathe It is also termed Common 

 Air, and Atmospheric Air, to distinguish 

 it from the other gases. See Gas and 

 Atmosphere. 



, IN A GENERAL SENSE, is any 



permanently elastic fluid which is so si- 

 milar, in this and other qualities, to com- 

 mon air, as to be properly classified under 

 the same general name. 'See Gas. 



, CONDENSED, is air rendered more 



dense by being subjected to pressure. 



, ETHEREAL, or ETHER, is an 



imaginary fluid, supposed by some to fill 

 all space beyond the atmospheres of the 

 eartu and other planets. 



, PRESSURE OF, a term sometimes 



B 



